The detection of ultrasound is commonly performed with ultrasound detectors using piezo-electric materials, which convert pressure fields into voltage. Such detectors are characterized by relatively high sensitivity, small size, low cost, flexible design, wide bandwidth, high portability, and the ability to be multiplexed. Currently, piezo-electric detectors are the method of choice for most ultrasound-based bio-medical imaging applications. However, thermoacoustic imaging, and specifically intravascular multispectral optoacoustic imaging may pose stringent requirements on the size and sensitivity of the ultrasound detectors which cannot be met by current piezo-electric detectors. In addition, the vulnerability of piezo-electric detectors to electromagnetic radiation poses difficulties for thermoacoustic-imaging applications.
One application where better acoustic detection is required is intravascular multispectral optoacoustic imaging. Early implementations of such systems relied on the use of an intra-vascular ultrasound (IVUS) detector for measuring the acoustic fields, which is based on piezo-electric technology. However, the miniaturization of the acoustic detector, which allows using IVUS in the coronary arteries, leads to diminished acoustic sensitivity. The exciting optoacoustic pulses should be sufficiently powerful such that the magnitude of the subsequent acoustic signal is higher than the detector's noise floor. However, because of safety restrictions pulses with high energy must be used in low repetition rates on the order of 10 Hz to ensure that the average laser power is below the tissue damage threshold. At such repetition rates, a single volumetric multi-spectral image with a typical length of several centimeters may take many hours to acquire. Such long acquisition times are unacceptable in catheterization procedures because they are invasive. To perform the data acquisition in an acceptable time, the acoustic-detector sensitivity should be improved by orders of magnitude.
Optical detection of acoustic fields has been previously proposed as an alternative to piezo-electric technology. Optical detection schemes are based on the photoelastic effect, where stress or strain in the optical medium leads to changes in its refractive index. The changes in the refractive index are commonly detected using interferometry. Optical detection schemes have been demonstrated for optoacoustic imaging [Beard, P C, and Mills, T N (1997): A miniature optical fiber ultrasonic hydrophone using a Fabry Perot polymer film interferometer, Electronics Letters 33(9), 801-803.]. In contrast to piezo-electric detectors they are not sensitive to external electromagnetic fields and thus are unaffected by the high intensity optical pulses or RF radiation coupled to the imaged object. In addition, since such optical detectors are transparent, they do not block the way of the high-power pulsed beam, allowing for more flexible geometries. However, current optical detectors for ultrasound fail to achieve either the bandwidth or sensitivity of which piezo-electric technology is capable. In addition, multiplexing of sensitive detectors has not been demonstrated. Thus, so far optoacoustic imaging systems which are based on optical detectors for the ultrasound signals did not show significant improvement over more conventional piezo-electric-detector designs in terms of imaging speed and fidelity.
Optical detection schemes proposed for optoacoustic imaging used either 2-beam interferometry (Mach Zehnder or Michelson) [Horacio Lamela et al. “Optoacoustic imaging using fiber-optic interferometric sensors,” Optics Letters, Vol. 34, 2009, page 3695-3697] or a Fabry-Perot structure [Beard, P C et al. (1997): A miniature optical fiber ultrasonic hydrophone using a Fabry Perot polymer film interferometer, Electronics Letters 33(9), 801-803]. Fiber Bragg Gratings (FBG's) have been demonstrated for the detection of low frequency ultrasound signals [N. E. Fisher et al. “Ultrasonic hydrophone based on short in-fiber Bragg gratings,” Applies Optics, Vol. 34, 1998, pp. 8120-8128]. The limited detection bandwidth, which prevents the use of such schemes in optoacoustic applications, was a result of the relatively large effective size of the detector. In addition, the interrogation was performed using a wideband continuous-wave source. The statistical properties of the light emitted by such sources were analyzed in [C. Z. Shi et al. “Noise Limit in Heterodyne Interferometer Demodulator for FBG-Based Sensors,” JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 22, NO. 10, OCTOBER 2004, pp. 2287-2295]. Although a quantitative comparison to other sources was not given, it can be shown that interrogation techniques based on a wideband continuous-wave source have an inherently high level of noise which severely limits sensitivity. The reason for this property is that the wide bandwidth of the source is a result of the incoherent nature of the source, i.e., fast random variations in the phase and amplitude of the emitted light. Other wideband-cw-source interrogation techniques can be found in [A. D. Kersey, J. Lightwave Tech. 15, 1142 (1997).]. These techniques are general and can be used to interrogate any linear optical device which exhibits a bandpass spectrum.
If the Bragg grating in an optical fiber includes a π-phase-defect in periodicity, a so-called π-phase-shifted FBG (Pi-phase-shifted FBG) is provided. The π-phase-shifted FBG is characterized by a localized-light resonance portion being formed around the defect and having spectral properties which depend on a mechanical strain exerted on the optical fiber. π-phase-shifted FBG's have been previously demonstrated as load sensors [D. LeBlanc et al. “Transverse load sensing by use of pi-phase-shifted fiber Bragg gratings,” OPTICS Letters, Vol. 24, 1999, page 1091], but have not been used for ultrasound detection. The interrogation was performed by inspecting the spectral splitting of the notch in the bandgap spectrum.
U.S. Pat. No. 7,206,259 discloses an ultrasound detector using a fiber sensor with FBG's. The fiber is an active medium, i.e., amplifies the guided light, and with the gratings leads to lasing. The laser light is affected by the acoustic fields. An optical ultrasound receiver is described in US 2010/0087732 A1, wherein a fiber sensor with FBG's is used. The FBG's are used as partially reflecting mirrors to form a Fabry Perot interferometer. The interrogation is performed with narrow-band CW illumination. U.S. Pat. No. 6,839,496 discloses a similar optical fiber probe for photoacoustic material analysis being adapted for forward viewing. Side-viewing optical acoustic sensors and their use in intravascular diagnostic probes are described in US 2007/0291275 A1. The optical sensor is a Fabry Perot interferometer. An ultrasound detection system, and material monitoring apparatus and nondestructive inspection apparatus equipped the system is described in US 2008/0043243 A1, wherein a fiber sensor with FBG's is used. The interrogation is done with a wide-band source.
It could therefore be helpful to provide an improved ultrasound detector avoiding disadvantages of conventional detector techniques such as high-sensitivity optical detection of ultrasound fields which is stable against environmental changes such as temperature drifts, pressure variations, and low frequency mechanical vibrations. Furthermore, it could be helpful to provide an improved ultrasound detecting device and an improved imaging apparatus, in particular for optoacoustic or thermoacoustic imaging an object under investigation such as a high-speed multi-spectral optoacoustic or thermoacoustic device implementation adapted for intra-vascular imaging. It could still further be helpful to provide optoacoustic or thermoacoustic imaging device implementations for non-invasive imaging.